Because of the favourable combination of low density, attractive elevated-temperature properties and acceptable fabricability, gamma titanium aluminides are emerging as revolutionary engineering materials to replace heavier nickel-base superalloys, steels and conventional titanium alloys for gas turbine and automotive applications with service temperatures of about 600.degree. C. to 800.degree. C. In recent years, tremendous research and development efforts have been made in alloy modification and microstructural control to improve mechanical properties as well as fabricability of the materials.
Gamma titanium aluminides based on TiAl phase usually contain about 45 to 49 atomic percent Al and are frequently referred to as near-gamma titanium aluminides. The constituents of the alloys normally consist of a predominant amount of TiAl (gamma) phase and a relatively minor amount of Ti.sub.3 Al (alpha-2) phase. FIG. 1 is the central portion of a titanium-aluminum phase diagram. In some multi-component alloys, a small volume fraction of titanium beta phase may also exist due to the presence of beta-stabilizing elements such as Cr, W, Mo, etc. Gamma alloys are typically produced by casting, thermomechanical processing or P/M processing, and heat treatments are usually employed to control the final microstructure of the product. The conventional heat treatments applied to gamma alloys typically involve a treatment at a temperature above T.sub..alpha. (line a-b in FIG. 1) or between T.sub..alpha. and the eutectoid temperature (line c-d in FIG. 1, .apprxeq.1125.degree. C.) for about 0.5 to 5 hours, followed by a secondary treatment at a temperature between 750.degree. C. and 1050.degree. C. for 4 to 100 hours to stabilize the heat treated microstructure. The cooling method used in the heat treatments can be furnace cooling, air cooling, or controlled cooling at a pre-determined rate, depending on the microstructural requirements. The typical microstructures produced by the conventional heat treatments include near gamma (NG), duplex (DP), nearly lamellar (NL), and fully lamellar (FL) structures.
Conventional processes of the type described above are exemplified in U.S. Pat. No. 5,226,985 to Kim et al. and U.S. Pat. No. 5,296,055 to Kenji.
For a given alloy composition, previous studies have shown that relatively good room-temperature tensile strength and ductility can be obtained in a duplex microstructure consisting of small equiaxed gamma grains and lamellar grains containing alternate gamma and alpha-2 lamellae. However, the room-temperature fracture toughness and elevated-temperature creep resistance of the duplex microstructure are poor. On the other hand, a fully lamellar microstructure composed of coarse lamellar grains offers much better fracture toughness and creep resistance, but unfortunately, with a substantial reduction in tensile strength and ductility. In comparison, a nearly lamellar microstructure containing predominantly large lamellar grains and a small amount of equiaxed fine gamma grains provides improved fracture toughness and creep resistance, with minimal loss in tensile property. However, the degree of improvement achieved in balancing these properties is largely dependent on the volume fraction of the equiaxed gamma grains, which appears to be difficult to control using conventional heat treatments.
Recent investigations have shown that the balance of mechanical properties for gamma alloys can be enhanced by reducing the grain size in a fully lamellar microstructure. This is because the refined grain size increases tensile strength and ductility, whereas the retained lamellar structure as well as the interlocking grain boundary morphology, associated with the lamellar structure, are beneficial for fracture toughness and creep resistance.
However, it has proven difficult to reduce the lamellar grain size solely by conventional heat treatment, and therefore several other methods have been recently developed. These methods include: (a) alloy modification, (b) thermomechanical processing (TMP) or thermomechanical treatment (TMT), or (c) XD.TM. (a trademark of Martin Marietta) processing. Each of these methods has advantages and limitations. Wrought gamma alloys that are compositionally modified with boron additions or large amounts of beta stabilizing elements can be heat treated in either an extended alpha plus beta two-phase region or in the alpha single-phase region with the presence of boride particles used to yield a fine-grained lamellar microstructure. However, this process is not applicable to many existing alloys which do not contain boron or large amounts of beta stabilizing elements. TMP and TMT are effective in refining the lamellar grain size in wrought alloys, however, the processes cannot be employed to refine the coarse microstructure of investment castings. Finally, XD.TM. processing yields a fine-grained cast lamellar microstructure through in-situ formation of TiB.sub.2 particles which act as nuclei for grain formation during solidification. The larger the number of such nuclei, the smaller the resulting grain size that will be produced in the fully solidified product. However, this process is limited to alloys that contain in-situ TiB.sub.2 particles and is not applicable to non-XD.TM. cast alloys.
Given the limitations of the above methods, it is an object of the present invention to provide a method for producing fine-grained lamellar microstructures in certain forms of gamma or near-gamma titanium aluminides, including powder metallurgy, wrought and cast alloys.